Variable attenuation signal acquisition probing and voltage measurement systems using an electro-optical cavity

ABSTRACT

A variable attenuation signal acquisition probing system and voltage measurement system uses an optical cavity to acquire a signal under test. The probing system has an optical transmitter and receiver that are coupled to the optical cavity via an optical transmission system. The optical cavity has an electrode structure having apertures formed in the optical cavity that are parallel to propagation path of the optical signal within the cavity. A modulated optical signal is generated by the optical cavity in response to the signal under test creating an electromagnetic field distribution in electro-optic material in the optical cavity that overlaps the optical path of the optical signal propagating in the optical cavity which varies the index of refraction of electro-optic material in the optical path. Changes in the polarization state of the optical signal attenuates the magnitude of the output electrical signal of the optical receiver.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the U.S. Provisional ApplicationNo. 60/552,334, filed Mar. 10, 2004.

BACKGROUND OF THE INVENTION

The present invention relates generally to variable attenuation signalacquisition probes and more particularly to variable attenuation signalacquisition probing systems using electro-optical cavities that areincorporated into voltage measurement systems.

Electro-optic material is a class of inorganic and organic crystalswhere the index of refraction of the material changes in response toelectro-magnetic energy applied to the material. Such material may beused in the production of optical devices, such as optical switches,optical limiters, optical modulators and the like. In it simplest form,an optical signal, such as the output of a laser or the like, islaunched into the electro-optic material having length and widths in themillimeter range and thicknesses in the tenths of millimeter range. Thediameter of the optical path of the optical signal within theelectro-optic material generally ranges from ten to a few hundredsmicrons across. Electrodes are formed on opposing surfaces of theelectro-optic material that are parallel to the optical path of thesignal passing through the electro-optic material. An electrical signalis applied to the electrodes which varies the index of refraction of theelectro-optic material as a function of the variations of the electricalsignal. The variations of the index of refraction of the electro-opticmaterial alters the optical signal propagating through the electro-opticmaterial.

Optically reflective material may be disposed on opposing sides of theelectro-optic material to form an optical cavity. A Fabry-Perot etalonis an example of such an optical cavity. The reflectivity of theoptically reflective material on the opposing sides of the electro-opticmaterial is defined by the particular application of the optical cavity.The optical signal passes through at least one of the opticallyreflective materials and into the electro-optic material. Electrodes areformed on opposing surfaces of the electro-optic material that areparallel to the optical path of the optical signal. An electrical signalapplied to the electrodes varies the index of refraction of theelectro-optic material as a function of the variations in the electricalsignal.

The strength of the electric field distribution within the electro-opticmaterial is a function of the distance between the opposing electrodesand the amplitude of the applied electrical signal. The strength of theelectric field is the inverse of the distance separation of theelectrodes. As the distance between the electrodes decreases, thestrength of the electric field between them increases. As the distancedecreases, the magnitude of the electrical signal can decrease togenerate the same amount of change in the index of refraction.

Currently, the minimum overall dimensions of the electro-optic materialused in optical devices and cavities is limited by the practical size atwhich the material can be handled resulting in electrodes that arepositioned at a substantial distance from the optical path of theoptical signal. This results in optical devices having low sensitivityto the applied electrical signal.

There is an increasing need in the electronics industry for measurementtest equipment, such as oscilloscopes, logic analyzers and the like, tomeasure electrical signals in the gigahertz range. Correspondingly,there is a need for measurement instrument signal acquisition probesthat have the signal bandwidth to acquire such high frequency signals.Generally gigahertz bandwidth signal acquisition probes have activecircuitry in the probing head of the probe that receives the electricalsignal via a metal probing tip extending from the end of the probinghead. Extensive design work is required to minimize probe tip inductanceand capacitance that affect the overall bandwidth of the probe. Inaddition, the dielectric constant of the probe head material also needsto be minimized for gigahertz differential signal acquisition probes. Afurther complication for gigahertz signal acquisition probe designs isthe signal loss through the coaxial cable that couples the probing headto the measurement instrument. Additionally, some measurement probesprovide varying amounts of attenuation to the measured electricalsignal. Generally, the measurement probe provides a set amount ofattenuation to the measured signal, such as 1×, 10×100×. The attenuationis added into the measurement probe circuit by the inclusion of avariable attenuator that is changes using manual or electronicswitching.

U.S. Pat. No. 5,808,473, titled “Electric Signal Measurement ApparatusUsing Electro-Optic Sampling by One Point Contact” describes anelectro-optic sampling high-impedance probe exploiting the Pockelseffect to rotate the polarization state of a light beam. The Pockelseffect changes the birefringence of an electro-optic crystal by anamount that is proportional to an electric field inside the crystal.With the proper application of electrodes to the crystal surface, andtheir connection to conductive probing tips, the polarization rotationcan be made to respond to a voltage on a device under test (DUT). Theelectro-optic sampling high-impedance probe receives polarizationmaintained laser pulses via a single mode polarization maintainingfiber. The laser pulses are coupled through bulk optic devices onto anelectro-optic element having a reflective film on one end. A metal pinin the end of the signal probe head abuts the reflective film on theelectro-optic element. The metal pin couples an electrical signal from adevice under test to the electro-optic element which alters thebirefringence of the electro-optic element in response to the electricalfield of the signal causing the polarization state of the laser beam tochange. The laser beam having the changed polarization state isreflected by the reflecting film and coupled through polarization beamsplitters which convert the S and P polarized beams into an intensitychange. The S and P polarized beams are coupled through respectivecondensing lenses onto respective slow germanium photodetectors thatconvert the optical beams into electrical signals. The electricalsignals are coupled to a measurement instrument and detected by adifferential amplifier.

U.S. Pat. No. 6,166,845 describes a modification to the above describedelectro-optic sampling high-impedance probe. Instead of coupling laserpulses via a single mode polarization maintaining fiber to the probe, alaser diode is incorporated into the probe itself. The laser diodegenerates a pulsed laser output in response to an input pulse chain fromthe measurement instrument. The probe contains the bulk optic devices,electro-optic element and photodetectors as previously described. Themetal pin couples the electrical signal from a device under test to theelectro-optic element which alters the birefringence of theelectro-optic element in response to the electrical field of the signalcausing the polarization state of the laser beam to change. The S and Ppolarized beams are coupled through the beam splitters and thecondensing lenses onto the photodetectors. The photodetectors convertthe intensity beams into electrical signals and couple the electricalsignals to the measurement instrument.

A drawback to this type of probe is the size of the probing head due tothe number of optical elements contained therein. Further, voltage andsignal lines are required to couple the voltage power to the laser diodeand photodetectors, couple the drive signal to the laser diode and tocouple the outputs of the photodetectors to the measurement instrument.

U.S. Pat. No. 5,353,262 describes an ultrasound optical transducer thatgenerates an optical signal the frequency of which varies incorrespondence with acoustic energy incident on the transducer. Thetransducer includes a housing in which is disposed a signal laser. Thesignal laser is preferably a microchip laser, microcavity laser or thelike. The signal laser has an optical cavity disposed between first andsecond reflectors and in which a lazing medium (also known as a gaincrystal) is disposed. The reflectors are disposed on opposingplane-parallel surfaces of the lasing medium. An optical source injectsan optical signal at a first frequency into the signal laser, whichgenerates a second output signal at a second frequency. Acoustic energyimpinging on the transducer causes the index of refraction of theoptical cavity to change which in turn, causes the frequency of thesignal laser to change. The frequency modulated optical signal from thesignal laser is coupled to signal processing assembly that generates anoutput signal corresponding to the amplitude of the incident acousticenergy for use in imaging and analysis. An alternative embodiment isdescribed where a piezoelectric device is positioned on the transducerfor converting the acoustic energy into an electrical signal. Theelectrical signal is applied to electrodes on the signal laser. Theelectrical signal causes a change in the index of refraction of theoptical cavity as a function of the acoustic energy applied to thepiezoelectric device.

U.S. Pat. No. 4,196,396 describes the use of a Fabry-Perot enhancedelectro-optic modulator to produce a bistable resonator that could beused as an optical switch, optical limiter, or optical memory device. Afurther embodiment taught by the '396 patent is an optical amplifier.The reference teaches the use of high voltage signals in the thousandvoltage range to change the index of refraction of the electro-opticmaterial in the Fabry-Perot cavity. Such a system does not lend itselffor small signal probing applications.

U.S. Pat. No. 5,394,098 describes the use of longitudinal Pockels effectin an electro-optic sensor for in-circuit testing of hybrids andcircuits assembled on circuit boards. In one embodiment, a layer ofelectro-optic material is disposed between opposing layers of opticallyreflective materials that include electrically conductive layers. Theoptically reflective layer having highest reflectivity to an appliedoptical signal is placed in contact with a conductor on the circuitboard. The other optically reflective layer is coupled to electricalground. An optical signal from a laser is applied orthogonal to theoptically reflective layers on the electro-optic material. An electricalsignal on the conductor of the circuit board produces a voltagepotential difference across the optically reflective layers which variesthe refractive index of the electro-optic material. A drawback to thisdesign is that the orientation of the polarized optical signal isorthogonal to the orientation of the electromagnetic field producing thePockels effect in the electro-optic material. This reduces thesensitivity of the measured electrical signal. Further, formingelectrically conductive layers on the opposing sides of theelectro-optic material produces capacitive and inductive effects in theelectro-optic sensor that limits the useful bandwidth of the system.

What is needed is a variable attenuation signal acquisition probingsystem using an electro-optical cavity that improves the sensitivity ofthe electro-optical cavity to applied electrical signals. Further, thereis needed a voltage measurement system using a variable attenuationsignal acquisition probing system with an electro-optical cavity withimproved sensitivity.

SUMMARY OF THE INVENTION

Accordingly, the present invention is a variable attenuation signalacquisition probing system usable in a voltage measurement system forsensing an electrical signal from a device under test. The variableattenuation signal acquisition probing system has an optical transmittergenerating a variable polarized, tunable, coherent optical signal and anoptical receiver generating an output electrical signal. An opticaltransmission system optically couples the optical signal from theoptical transmitter to an optical cavity and couples a modulated opticalsignal from the optical cavity to the optical receiver. The opticalcavity has optically reflective material disposed on opposing surfacesof an electro-optic material with the tunable, coherent optical signalpropagating through at least one of the optically reflective materialsand within the electro-optic material. First and second electricallyconductive electrodes receive the electrical signal from the deviceunder test. Each electrically conductive electrode has an aperturesformed in at least a portion of the electro-optic material generallyparallel to the received optical signal propagating within theelectro-optic material with each electrode having electricallyconductive material is disposed therein. The modulated optical signal isderived from the device under test electrical signal creating anelectro-magnetic field distribution in the electro-optic material thatoverlaps the optical path of the optical signal propagating in theelectro-optic material which varies the index of refraction of theelectro-optic material in the optical path. Control circuitry controlsthe optical power level and wavelength of the tunable, coherent opticalsignal from the optical transmitter and the gain of the outputelectrical signal from the optical receiver. A variable polarizerreceives the tunable, coherent optical signal and varies thepolarization state of tunable, coherent optical signal to change theattenuation level of the output electrical signal of the opticalreceiver.

The electro-optic material has X, Y, and Z optical axes andcorresponding crystal faces orthogonal to the respective X, Y, and Zoptical axes. The optically reflective materials may be disposed on theopposing crystal faces orthogonal to one of the X, Y, and Z opticalaxis. The received optical signal propagates generally parallel to atleast one of the optical axes in the electro-optic material with thefirst and second electrically conductive electrodes generally parallelto same optical axis. Electrically conductive contacts may be formed onan at least one exterior surface of the optical cavity with the one ofthe electrically conductive contacts electrically coupled to the firstelectrically conductive electrode and the other electrically conductivecontact electrically coupled to the second electrically conductiveelectrode. Additionally, a resistor may be coupled between theelectrically conductive electrodes or between the electricallyconductive contacts. An acoustic damping material covers a substantialportion of the optical cavity to minimize acoustic modes in the opticalcavity. In the preferred embodiment of the invention, the optical cavitycomprises a Fabry-Perot optical cavity.

The optical transmission system may be implemented with an opticaldirectional coupler having a first port optically coupled to the opticaltransmitter, a second port optically coupled to the optical receiver anda third port optically coupled to one end of an optical fiber. The otherend of the optical fiber optically is coupled to one of the opposingoptically reflective materials of the optical cavity. In the preferredembodiment, a collimating lens is optically coupled to the optical fiberwith the collimating lens disposed adjacent to one of the opposingoptically reflective materials of the optical cavity. When the signalacquisition probing system uses a Fabry-Perot optical cavity, theoptical directional coupler is a polarization maintaining opticaldirectional coupler with the first port optically coupled to the opticaltransmitter via a polarization maintaining optical fiber and the thirdport coupled to the collimating lens via a polarizing maintainingoptical fiber. The optical transmission system may further beimplemented with a polarizing maintaining optical fiber opticallycoupling the optical transmitter to the collimating lens and an opticalfiber optically coupling the collimating lens to the optical receiver.

The optical receiver may be implemented with at least a firstoptical-to-electrical converter coupled to receive the modulated opticalsignal from the optical cavity via the optical transmission system. Theoptical receiver generates an electrical signal representative of theelectrical signal from the device under test which is amplified by anamplifier. In the preferred embodiment, the optical receiver has anoptical beam splitter receiving the modulated optical signal from theoptical cavity. The beam splitter optically couples a first portion ofthe modulated optical signal to the first optical-to-electricalconverter and a second portion to a second optical-to-electricalconverter. The second optical-to-electrical converter generates anelectrical signal that is coupled to the control circuitry for varyingthe wavelength of the coherent optical signal of the optical transmitterto maintain an optimum modulated reflected power from the opticalcavity. Preferably, greater than ninety percent of the modulated opticalsignal is coupled to the first optical-to-electrical converter and lessthan ten percent to the second optical-to-electrical converter.

The optical transmitter, optical receiver and the control circuitry maybe disposed in a probe interconnect housing wherein the measurementinstrument and the probe interconnect housing have a common interface.The interface provides coupling of the electrical signal from the deviceunder test to the measurement instrument, the coupling communicationsdata between the measurement instrument and the signal acquisitionprobing system and the coupling electrical power to the signalacquisition probing system from the measurement instrument. The opticaltransmitter, optical receiver and control circuitry may also be disposedin a separate probe controller having a micro-controller and powersupply therein. The micro-controller receives inputs for controlling theoperations of the optical transmitter, optical receiver and the controlcircuitry and the power supply provides electrical power the optical andelectrical circuits. The probe controller has optical and electricaloutput connectors for coupling the electrical signal from the probecontroller to the measurement instrument via an electrical cable andcoupling the optical signal from the optical transmitter to the opticalcavity and a modulated optical signal to the optical receiver via theoptical transmission system. The combination of the signal acquisitionprobing system with the measurement instrument, such as an oscilloscope,logic analyzer, vector network analyzer or the like forms a voltagemeasurement system.

The objects, advantages and novel features of the present invention areapparent from the following detailed description when read inconjunction with appended claims and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate alternative electrode configurations of theelectrode structure for optical cavity used in a variable attenuationsignal acquisition probing system according to the present invention.

FIGS. 2A-2E illustrate alternative contact configurations for theelectrode structure in the optical cavity used in a variable attenuationsignal acquisition probing system according to the present invention.

FIG. 3A-3B illustrate alternative embodiments of the optical cavity inthe variable attenuation signal acquisition probing system according tothe present invention.

FIG. 4 illustrates a variable attenuation signal acquisition probingsystem incorporated into a voltage measurement system according to thepresent invention.

FIG. 5 general block diagram of the circuitry in the variableattenuation signal acquisition probing system according to the presentinvention.

FIG. 6 illustrates the resonate wavelengths for a Fabry-Perot opticalcavity used in the variable attenuation signal acquisition probingsystem according to the present invention.

FIG. 7 illustrates more detailed block diagram of the circuitry in thevariable attenuation signal acquisition probing system according to thepresent invention.

FIG. 8 illustrates an analog implementation of the bias and controlcircuitry in the variable attenuation signal acquisition probing systemaccording to the present invention.

FIG. 9 illustrates a partially sectioned view of the probing componentswithin the probing head in the variable attenuation signal acquisitionprobing system according to the present invention.

FIG. 10 illustrates a further embodiment of the voltage measurementsystem according to the present invention incorporating a furtherembodiment of the variable attenuation signal acquisition probingsystem.

FIG. 11 is a block diagram of the probe controller in the variableattenuation signal acquisition probing system according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1A, 1B and 1C, there are shown various electrodesstructures 10 usable in an optical cavity 12 incorporated into thevariable attenuation signal acquisition probing system of the presentinvention. The variable attenuation signal acquisition probing systemwill be described in relation to a Fabry-Perot optical cavity but otheroptical cavities incorporating the electrode structure 10 may be usedwith the variable attenuation signal acquisition probing system. Theoptical cavity 12 has an electro-optic material 16 disposed betweenopposing optically reflective materials 18 and 20. The electro-opticalmaterial may be formed from inorganic and organic materials, such asPotassium Titanyl Phosphate (KTP), Rubidium Titanyl Arsenate (RTA),Rubidium Titanyl Phosphate (RTP), Zinc Telluride (ZnTe),DimethylAmino-methyl Stilbazolium Tosylate (DAST) or other electro-opticmaterials, such as electro-optic polymers, all having the property of achanging index of refraction in response to an applied electro-magneticfield. The inorganic and organic materials have crystallographic axesdefining the crystallographic structure of the electro-optic material16. Crystals systems are cubic, tetragonal, orthorhombic, monoclinic andtriclinic. The crystallographic axes for the cubic, tetragonal and theorthorhombic systems are mutually perpendicular to each other. Themonoclinic and triclinic crystal systems have one or more of thecrystallographic axes at oblique angles to each other. The hexagonalcrystal system has two crystallographic axes falling on the same planeat 120° to each other and a third axis orthogonal to the other two. Theinorganic and organic materials further have X, Y and Z optical axeswhich may or may not coincide with the crystallographic axes.

The optical cavity 12 will be described below in relation to inorganicKTP electro-optic material having an orthorhombic crystalline structureand optical axes coincident with the crystallographic axes. It isunderstood that the optical cavity 12 is applicable to the other crystalstructures and organic polymers having one or more optical axes that areresponsive to an electromagnetic field for changing the index ofrefraction of the electro-optic material. Further, the present inventionwill be described in relation to specific optical axes of the KTPelectro-optic material 16 and a specific orientation of a propagatingoptical signal 14 and orientations of the electromagnetic field withinthe KTP electro-optic material 16. In the preferred embodiment, the KTPelectro-optic material 16 is an X-cut crystal face where the cleaved andpolished surfaces of the crystal are perpendicular to the opticalX-axis. Alternatively, the KTP electro-optic material 16 may be a Y-cutcrystal face. The X-cut crystal is preferred over the Y-cut crystal forminimizing distortions from the acoustic modes generated within theelectro-optic material 16. It should be noted that the electro-opticproperties of other crystallographic structures may result in thepreferred cut crystal face being orthogonal to the optical Z-axisproducing a Z-cut crystal face.

The optical signal 14 provided to the optical cavity 12 is preferablyprovided by a coherent optical source, such as a laser diode or thelike. The optical signal 14 is polarized as either linear or circularpolarized light. The optical signal preferably passes through bulk opticlenses to provide a generally collimated or focused beam onto theoptically reflective materials 18. An example of a generally collimatedoptical signal 14 focused on an electro-optic material is a 1310 nmoptical signal having an optical path diameter ranging fromapproximately 15 to 150 microns. Other optical path diameters may beused with the electrode structure of the present invention. The linearor circular polarization states of the optical signal 14 are normal tothe propagation direction of the signal. The lateral dimensions of theoptically reflective materials 18 and 20 should exceed the beam diameterof the optical signal 14 impinging on the optical cavity 12. In theembodiments of FIGS. 1A, 1B and 1C, the optically reflective materials18 and 20 generally conform to the diameter of the optical path and areformed on the X-cut crystal faces of the electro-optic material 16. Theoptically reflective materials 18 is partially reflective to allow theoptical signal 14 to enter and exit the optical cavity 12. In certainapplications the optical reflective material 20 is preferably totallyreflective causing the optical signal to enter and exit through the sameoptically reflective material 18. The optically reflective materials 18and 20 are preferably ceramic mirrors formed from layers of zirconiumdioxide, silicon dioxide and silicon nitride. It is important in certainapplications that the optically reflective materials be non-metallic toreduce capacitive and inductive effects.

The change in the index of refraction of the electro-optic material 16in the presence of an electromagnetic field is a function of theorientation of the optical signal propagating in the electro-opticmaterial 16 and the relationship of the polarization state of theoptical signal 14 and the electrode structures 10 to the optical axes ofthe electro-optic material 16. For example, KTP electro-optic materialexhibits the highest index of refraction and largest sensitivityresponse to an electro-magnetic signal when the polarization state ofthe optical signal 14 and the electromagnetic field are parallel withthe optical Z-axis of the KTP material. However, the KTP electro-opticmaterial exhibits the highest piezoelectric response along the Z-axis,and the lowest piezoelectric response along the X-axis, when theelectro-magnetic field is parallel to the optical Z-axis. Thepiezoelectric effect causes a change in the refractive index of thecrystal, but also physically alters the length of the material (orstrain) along the three principle crystal axes. To minimize the effectof the piezoelectric strain on the modulated signal, it is desirable toensure that the smallest change in crystal length occurs along thecrystal axis that is perpendicular to the two cavity mirrors attached tothe crystal. Therefore, in the preferred embodiment, the polarizationstate of the optical signal 14 and the electro-magnetic field areparallel with the optical Z-axis, and the optical beam propagatesthrough the crystal parallel to the X-axis to minimize the effects ofthe acoustic modes in the KTP electro-optic material on the resultingoptical modulation.

The electrode structures 10 in FIGS. 1A, 1B and 1C have a pair ofapertures 22 and 24 formed in the KTP electro-optic material 16 that aregenerally parallel to the optical path 26 of the received optical signal14 propagating through the electro-optic material 16. The KTPelectro-optic material 16 has mutually perpendicular optical axes X, Yand Z that coincide with the crystallographic axes of the KTP material.The apertures 22 and 24 are disposed on the opposite sides of theoptical path 26 of the propagating optical signal 14 and are orientedparallel to the optical X-axis of the electro-optic material 16. Theapertures 22 and 24 are preferably formed as close as possible to thepropagating optical signal 14 with the aperture separation, for example,being in the range of 45 to 120 microns. In some applications, theapertures 22 and 24 may extend into the optical path 26 of thepropagating optical signal 14. The apertures 22 and 24 in FIG. 1A have apolygonal sectional shape with an apex directed toward the optical path26 of the propagating optical signal 14. The apexes of the polygonalshapes concentrates the electromagnetic field across the optical path26, which is parallel to the optical Z-axis of the electro-opticmaterial. The polygonal electrode structure does not lend itself tousual manufacturing processes whereas a circular electrode structure asillustrated in FIG. 1B is easily produced. The circular apertures 22 and24 in FIG. 1B have the same orientation with the optical path as in FIG.1A. The circular apertures 22 and 24 are produced using an excimerpulsed laser that can produce apertures of approximately 100 microns indiameter and of varying depth in the electro-optic material 16. Thecircular apertures 22 and 24 in FIG. 1C are shown extending part waythrough the electro-optic material 16 and have the same orientation withthe optical path in FIG. 1B. The blind hole apertures reduce the risk ofdamage to the electro-optic material 16 when the pulsed laser light fromthe excimer laser reaches the opposite end of the optical cavity 12. Theaperture configurations of FIGS. 1A-1C are but three examples and otheraperture configurations are possible without departing from the scope ofthe invention.

Electrically conductive material 28 is disposed within each of theapertures 22 and 24. The electrically conductive material 28 may takethe form of conductive wires shaped to conform to the apertures 22 and24, conductive material deposited on the inner surfaces of theapertures, conductive epoxy filling the apertures, or the like. Thedeposited conductive material is preferably gold plated over a layer ofchromium. The electrically conductive material 28 preferably extends tothe exterior surface of the one of the electro-optic material 16 toallow the electrode structure 10 to be electrically coupled to anelectromagnetic source, such as a voltage source. Alternately, theelectrically conductive material 28 may be connecting terminals for thevoltage source where the ends of the terminals are inserted into theapertures 22 and 24. In a further alternative, the electricallyconductive material 28 may reside totally within the electro-opticmaterial 16 and the connecting terminals are inserted into the apertures22 and 24 to make contact with the electrically conductive material 28.Forming the electrode structure 10 within the optical cavity 12decreases the distance between the electrodes thus increasing thestrength of the electric field applied across optical path 26 of thepropagating optical signal 14. This increases the sensitivity of theelectro-optic material 16 to the applied electric field.

In a specific embodiment where the electrically conductive material 28is an electrically conductive epoxy, the apertures 22 and 24 extendthrough the optical cavity 12 and the electrically conductive epoxyfills the apertures 22 and 24. Filter paper is positioned on one side ofthe optical cavity 12 covering the apertures 22 and 24. A vacuum isapplied to this side of the optical cavity 12 and the electricallyconductive epoxy is applied to the apertures 22 and 24 on the other sideof the optical cavity 12. The vacuum causes the electrically conductiveepoxy to be drawn into the apertures 22 and 24. The filter paperprevents the electrically conductive epoxy from being drawn out of theapertures 22 and 24.

FIGS. 2A through 2E illustrate the optical cavity 12 having theoptically reflective materials 18 and 20 disposed over the opposingsurfaces of the electro-optic material 16. In such a configuration, theapertures 22 and 24 of the electrode structure 10 extend through a leastone of the optically reflective materials 18 and 20. FIGS. 2A through 2Eshow alternative electrically conductive contact 30 configurations inthe electrode structure 10 of the present invention. The electricallyconductive contacts 30 may be formed using well know depositiontechniques, such as thin and thick film processes. The electricallyconductive contacts 30 are preferably formed of gold deposited over alayer of chromium. In FIGS. 2A and 2B, the electrically conductivecontacts 30 are formed on the same exterior surface 32 of the opticallyreflective material 20 with each contact 30 in electrical contact withthe electrically conductive material 28 in one of the respectiveapertures 22 and 24. The electrically conductive contacts 30 arepreferably a polygonal shape with an apex electrically coupled to therespective electrically conductive materials 28 in the apertures 22 and24. In the preferred embodiment where the apertures are circular as inFIG. 2B, the separation between the electrically conductive contacts 30is in the range of 15 to 100 microns with the apertures 22 and 24 setslightly back from the apexes of the contacts 30. In FIGS. 2C and 2D,the electrically conductive contacts 30 are formed on opposing exteriorsurfaces 34, 36 and 38, 40 of the electro-optic material 16. Conductivetraces 42 electrically couple the electrically conductive material 28 ofthe respective apertures 22 and 24 to the electrically conductivecontacts 30 on the opposing surfaces 34, 36 and 38 and 40. While thefigures illustrate the electrically conductive contacts 30 being onopposing surfaces of the electro-optic material 16, the electricallyconductive contacts 30 may be formed on adjacent surfaces of theelectro-optic material 16. As with the electrically conductive contacts30 formed on the same surface, the apertures 22 and 24 intersect theconductive traces 42 with the separation between the conductive tracesat the apertures 22 and 24 being in the range of 15 to 100 microns. FIG.2E illustrates a further configuration for the electrically conductivecontacts 30. Apertures 44 are formed in the electro-optic material 16that intersect the respective electrode structure apertures 22 and 24.Electrically conductive contacts 30 are formed on the surface orsurfaces of the electro-optic material 16 that intersect the apertures44. Electrically conductive material 46 is disposed in the apertures 44that electrically couples the electrically conductive contacts 30 to theelectrically conductive material 28 in the apertures 22 and 24.

FIGS. 3A and 3B illustrate further embodiments of the optical cavity.The electrode structure 10 described has an high input impedance. Incertain applications it may be preferable to match the impedance of theelectrode structure 10 to the impedance of the device providing theelectromagnetic energy to the electrode structure 10. In FIG. 3A, anoptional termination resistor 50 is shown formed on exterior surface 32of the optical cavity 12 that is perpendicular to the apertures 22 and24. The termination resistor 50 is connected between the electricallyconductive materials 28 in the apertures 22 and 24 of the optical cavity12. The termination resistor 50 may be formed using well knownprocessing techniques, such as thin or thick film processing. Theresistance of the termination resistor 50 is set to match the impedanceof the device driving the optical cavity 12. The termination resistor 50may also be formed on exterior surface 52 of the optical cavity 12 wherethe apertures are formed as through holes in the electro-optic material.In FIG. 3B, the optional termination resistor 50 is shown connectedbetween the electrically conductive contacts 30 on the exterior surface32 of the optically reflective material 20. In the embodiments whereconductive traces 42 couple the electrically conductive contacts to theelectrically conductive materials 28 in the apertures 22 and 24, thetermination resistor 50 may be coupled to the conductive traces 42.

Referring to FIG. 4, there is illustrated a variable attenuation signalacquisition probing system 90 coupled to a measurement instrument 92,such as real-time or sampling oscilloscopes, logic analyzer, vectornetwork analyzer, or the like. The variable attenuation signalacquisition probing system 90 has a probing head 94 containing theoptical cavity 12 and an optical transmission system 96 extending fromthe probing head 94 to a probe interconnect housing 98. The probeinterconnect housing 98 contains variable attenuation signal acquisitionprobing circuitry needed to provide an optical signal to the probinghead 94 and convert the returning modulated optical signal to anelectrical signal. The optical transmission system 96 preferablyincludes one or more optical fibers. The probe interconnect housing 98is removably connected to one of several interconnect receptacles 100 onthe front panel 102 of the measurement instrument 92. The probeinterconnect housing 98 and interconnect receptacles 100 are preferablyTekConnect® interface devices such as described in U.S. Pat. No.6,402,565 and incorporated herein in its entirety by reference. TheTekConnect® interface has connections for coupling a wide bandwidthsignal to measurement instrument, providing electrical power from themeasurement instrument 92 to the probe interconnect housing 98 andcommunication signals between the measurement instrument 92 to the probeinterconnect housing 98 as described in U.S. Pat. No. 6,629,048 andincorporated herein in its entirety by reference. The electrical signalrepresenting the measured signal from the device under test 104 iscoupled to acquisition circuitry within the measurement instrument 92that converts the electrical signal into digital data values and storesthe data values in memory. Processing circuitry operating under programcontrol processes the digital data values to produce display data thatis displayed on a display device 106, such as a liquid crystal display,cathode ray tube or the like. Alternately, the measurement instrument 92may include the variable attenuation signal acquisition probingcircuitry. The probe interconnect housing 98 would then include one ormore optical connectors for coupling the optical signal to the probinghead 94 and the return modulated optical signal to the measurementinstrument 92.

FIG. 5 is a general block diagram of the signal acquisition probingcircuitry 110 disposed in the probe interconnect housing 98 for aprobing head 94 having a Fabry-Perot optical cavity functioning as avoltage signal sensor. The Fabry-Perot optical cavity as used in thisembodiment has a Free Spectral Range of 2-4 nanometers using KTPelectro-optic material having an index of refraction of 1.86 parallel tothe optical Z-axis and a thickness along the optical X-axis of 0.1 to0.2 millimeters. The Fabry-Perot optical cavity has multiple resonancesdefined by the Free Spectral Range. FIG. 6 illustrates the resonatewavelengths for the above described Fabry-Perot optical cavity with thehorizontal axis in wavelength and the vertical axis in the normalizedreflected power of the Fabry-Perot optical cavity at the input to anoptical receiver. As is shown in the graph, the reflected optical powerdrops steeply from one-hundred percent reflected optical power toessentially zero percent optical power at the resonance points. Optimummodulated reflected power from the Fabry-Perot optical cavity isachieved on the slope of the resonance curve. This characteristic of theFabry-Perot optical cavity is used in the implementation of the variableattenuation signal acquisition probing system 90. The Fabry-Perotoptical cavity generates a modulated optical signal in response to ameasured electrical signal from the device under test 104.

Returning to FIG. 5, the signal acquisition probing circuitry 110includes an optical transmitter 112, optical receiver 114 and controlcircuitry 116 for the optical transmitter 112 and receiver 114. Theoptical output from the optical transmitter 112 and the optical input tothe optical receiver 114 may be coupled directly to the probing head 94via individual optical fibers 118 and 120 bundled in the opticaltransmission system 96. To maintain the polarization state of theoptical signal from the optical transmitter 112, the optical fiber 118is a polarization maintaining optical fiber. Alternately, the output ofthe optical transmitter 112 and the input to the optical receiver 114may be optically coupled to ports on an optical directional coupler 122with a single polarization maintaining optical fiber connecting theoptical directional coupler 122 to the probing head 94. The opticaldirectional coupler 122 is a polarization maintaining optical coupler tomaintain the polarization state of the optical signal from the opticaltransmitter 112. Data/control and voltage power lines 124 couple thesignal acquisition probing circuitry in the probe interconnect housing98 to the measurement instrument 92. A high speed coaxial interconnect126 couples the electrical signal from the optical receiver 114 to themeasurement instrument 92.

FIG. 7 is a more detailed block diagram of the signal acquisitionprobing circuitry 110 in the probe interconnect housing 98. The opticaltransmitter 112 is preferably a laser diode 130 generating an opticaloutput having a wavelength of approximately 1310 nm. The laser diode 130includes a thermoelectric (TE) cooler and thermistor for controlling thewavelength of the laser output and a photodetector for generating anelectrical output representative of the magnitude of the laser output.The output of the laser diode 130 is optically couple to a variablepolarizer 131. The output of the variable polarizer 131 is opticallycoupled to the probing head 94 via the optical transmission system 96.The optical receiver 114 has a beam splitter 132 receiving the modulatedoptical output from the probing head 94 via the optical transmissionsystem 96. The beam splitter 132 preferably has a splitting ratio ofgreater than 10 to 1 with the majority of the optical signal beingcoupled to an optical-to-electrical converter (O/E) 134, such as a PINor avalanche photodiode, that has a good response (sensitivity) to thewavelength of the output laser 130. The O/E converter 134 converts thereturn modulated optical signal to an amplitude modulated electricalsignal representative of the signal being measured on the device undertest 104. The electrical signal from the O/E converter 134 is amplifiedby amplifier circuitry 136 and coupled via the coaxial interconnect 126to the measurement instrument 92. The O/E converter 134 and amplifiercircuitry 136 form a high speed optical receiver for the measured signalfrom the device under test 104. The smaller portion of the opticalsignal from the beam splitter 132 is coupled to a second O/E converter138 that converts the optical signal to an electrical signal. The O/Econverter 138, such as a PIN or avalanche photodiode, has a goodresponse (sensitivity) to the wavelength of the output laser 130. Thesecond O/E converter 138 functions as a low speed device that producesan electrical signal representative of the average reflective power fromthe Fabry-Perot cavity.

The control circuitry 116 includes bias and thermo-electric (TE) controlcircuitry 140 and 142 for maintaining the laser output at a constantlevel and at an optimum wavelength for maximum modulated reflected powerfrom the Fabry-Perot optical cavity. The bias and TE control circuitry140 and 142 as well as the variable polarizer 113 are coupled to amicro-controller 144 via data and control bus 146. Serial data/controland voltage lines 124 provide communications between the measurementinstrument 92 and the micro-controller 144, and electrical power to theoptical transmitter and receiver and control circuitry 112, 114, 116. Ananalog-to-digital converter (A/D) 148 converts the electrical signalfrom the O/E converter 138 for processing by the micro-controller 144.The micro-controller may further be coupled a digital-to-analogconverter (D/A) 150 via the data and control bus 146 for controlling thegain of the amplifier circuitry 136. Additional electronicallycontrolled circuitry, such as variable attenuators, gain cells and thelike, may be incorporated into the output signal path of the opticalreceiver 114.

The micro-controller 144 has programmed command instructions stored inmicro-controller memory for controlling the operations of the varioussignal acquisition probe circuits 110. The bias control circuitry 140 inconjunction with the programmed command instructions in themicro-controller 144 provides a feedback loop to maintain the opticaloutput of the laser 130 at a constant level. The current output from thephotodetector in the laser 130 increase or decreases in response tochanges in the output power of the laser 130. The bias control circuitry140 samples the electrical signal from the photodiode in the laser 130and produces digital data values that are coupled to themicro-controller 144 via the data and control bus 146. The digital datavalues are processed by the micro-controller 144 to generate digitaldata values for driving a power amplifier. The processing may includeapplying scaling and calibration constants to the input digital datavalues to compensate for variations in the linearity of the laser outputlevel to applied bias levels. The digital data values for driving thepower amplifier are coupled to the bias control circuit via the data andcontrol bus 146 and converted to an analog signal for application to thepower amplifier. The output from the power amplifier is applied to thelaser diode 130.

The circuitry for controlling the wavelength of the laser 130 provides adual feedback loop having a local feedback loop within an overallfeedback loop to maintain the optical output of the laser at apredetermined wavelength. The first feedback loop is the local feedbackloop that includes the laser thermistor, the TE control circuitry 142,and the micro-controller 144. The second feedback loop includes theFabry-Perot optical cavity in the probing head, O/E converter 138, theA/D converter 148, the micro-controller 144 and the TE control circuitry142. The signal output from the thermistor in the laser 130 increase ordecreases in response to changes in the temperature of the laser 130.The TE control circuitry 140 samples the electrical signal from thethermistor in the laser 130 and produces digital data values. The outputfrom the O/E converter 138 representing the average reflected power fromthe Fabry-Perot optical cavity is converted to digital data values inthe A/D converter 148. The digital data values from the TE controlcircuitry 142 and the A/D converter 148 are coupled to themicro-controller 144 via the data and control bus 146. The digital datavalues are processed by the micro-controller 144 to generate digitaldata values for driving a TE cooler driver. The processing may includeapplying scaling and calibration constants to the input digital datavalues to compensate for variations in the linearity of the laser outputwavelength as a function of the laser temperature. The overall feedbackloop is controlled by the Fabry-Perot optical cavity transfer function.The thermistor digital data values provide a course adjustment controlfor the wavelength of the laser 130 while the A/D digital data valuesprovide a fine adjustment control for the laser wavelength. As shown inFIG. 6, the optimum output laser wavelength falls on either the negativeor positive-going slopes 152, 154 of the reflective power curve adjacentto the resonance wavelengths of the Fabry-Perot optical cavity. Themicro-controller 144 uses the thermistor digital data values to maintainthe output wavelength of the laser 130 at the optimum output laserwavelength for producing the optimum modulated reflected power from theFabry-Perot cavity for a measured electrical signal from the deviceunder test 104. The micro-controller 144 uses the A/D converter 148digital data values to continuously adjust the temperature of the laser130 to maintain the output of the laser at the optimum wavelength. Thedigital data values generated by the micro-controller 144 are coupled tothe TE control circuitry 142 via the data and control bus 146 andconverted to an analog signal for application to the TE cooler driver.The output from the TE cooler driver is applied the TE cooler in thelaser diode 130.

While the above control circuitry 116 has been described as amicro-controller based system, the circuitry may equally be implementedwith analog circuitry. Referring to FIG. 8, there is illustratedrepresentative analog bias control and TE control circuitry 140 and 142.In the analog implementation, the micro-controller 144 is decoupled fromthe bias and TE control circuitry 140 and 142. Further, the A/Dconverter 148 is removed so that the analog output of the O/E converter138 is coupled directly to the TE control circuitry 142. The biascontrol circuitry 140 has a voltage divider network 143 consisting ofresistors 145 and 147 that provides a reference level to thenon-inverting input of a positive gain driver amplifier 149. Theinventing input of the driver amplifier 149 receives the electricalsignal from the photodetector 151 in the laser 130. The reference levelfrom the intermediate node of the voltage divider network 143 sets theoutput of the drive amplifier 147 for driving the laser diode 153 at adesired power level. As the output of the photodetector 151 increasesand decreases with changes in the output power of the laser diode 153,the voltage applied to the inverting input of the drive amplifier 149varies the output of the drive amplifier 149. The drive amplifier 149varies the bias on the laser diode 153 to maintain the laser output at aconstant level.

The TE control circuitry 142 has a voltage divider network 155 thatincludes the thermistor 157 in the laser 130 coupled to a voltage sourceand resistor 159. The intermediate node of the voltage divider network155 is coupled to the non-inverting input of a TE driver amplifier 161.The inverting input of the TE driver amplifier 161 is coupled to receivethe electrical signal from the O/E converter 138. The output of the TEdriver amplifier 161 is coupled to the input of an integrating amplifier163. The output of the integrating amplifier 163 is coupled to the TEcooler 165 in the laser 130. The TE control circuitry 142 is designed toproduce the equal voltages on the inverting and non-inverting inputs ofthe TE drive amplifier 161 when the optimum output laser wavelengthfalls on either the negative or positive-going slopes 152, 154 of thereflective power curve adjacent to the resonance wavelengths of theFabry-Perot optical cavity. When the variable attenuation signalacquisition probing system is powered-up, the wavelength of the laser130 may not be on one of the positive or negative going slopes 152, 154of the reflective power curve. By including the integrating amplifier163 in the TE control circuitry 142, the wavelength of the laser 130 isdriven to one of the positive or negative slopes of the reflective powercurve adjacent to the one of the resonant wavelength of the Fabry-Perotoptical cavity. Once the laser is at the proper operating wavelength,any deviation from the optimum wavelength changes the average opticalpower reflected from the Fabry-Perot optical cavity. The O/E converter138 detects the change in the average optical power and generates anoutput that causes the voltage to the inverting input of the TE driveamplifier 161 to increase or decrease. This causes the output voltage ofthe TE drive amplifier to change which causes the integrating amplifier163 to produce a ramp signal. The ramp signal is applied to the TEcooler 165 in the laser 130 which causes the wavelength of the laser 130to increase or decrease accordingly.

As previously stated, the electro-optic material 16 has a preferredoptical axis exhibiting the highest index of refraction for an appliedelectromagnetic field. KTP electro-optic material exhibits this highestindex of refraction when the electromagnetic field is orthogonal to theoptical X-axis and parallel with the optical Z-axis of the KTP material.The largest sensitivity response to the applied electromagnetic field iswhen the polarization state of the optical signal 14 is parallel withthe optical Z-axis of the KTP material. The variable attenuation signalacquisition probing system 90 uses this property of electro-opticmaterial 16 to produce a probing system with variable attenuation wherethe attenuation switching is performed by changing the orientation ofthe polarization state of the optical signal to the Fabry-Perot opticalcavity in the probing head 94. The micro-controller 144 receives probeattenuation parameters from the measurement instrument via thedata/control lines 124 for setting the variable polarizer 113 to orientthe polarization state of the optical signal from the opticaltransmitter 112 parallel to the optical Z-axis of the KTP material. Thisproduces the greatest sensitivity response to the appliedelectromagnetic signal applied to the electrode structure 10. Settingthe probe attenuation parameters for increased attenuation results inthe micro-controller tuning the variable polarizer 113 to orient thepolarization state of the optical signal parallel to the optical Y-axisand orthogonal to the optical Z-axis which lowers the sensitivityresponse of the optical signal to the electromagnetic field applied tothe electrode structure 10.

FIG. 9 is a partially sectioned view illustrating the probing componentswithin the probing head 94. The probing components include a collimatinglens 160, such as manufactured and sold by Koncent under Part No.KPMT-A-400-1310-Y-0.5-G-N. The collimating lens 160 has a length ofapproximately 0.25 inches and a diameter of approximately 0.1 inches.The optical fibers 118 and 120 from the signal acquisition controlcircuitry 110 are disposed adjacent to the top flat surface 162 of thecollimating lens 160. A Fabry-Perot optical cavity 12 is secured to theopposing bottom surface 166 of the collimating lens using anon-conductive adhesive, such as epoxy or the like. The preferredstructure of the Fabry-Perot optical cavity 12 is essentially the sameas previously described.

The Fabry-Perot optical cavity 12 has the electrode structure 10 withelectrically conductive contacts 30 formed on the bottom exteriorsurface of the cavity 12. The reflective coating 18 and 20 on the topand bottom surfaces of the Fabry-Perot optical cavity 12 are formed ofthe previously described non-conductive materials. It is important inprobing applications to minimize conductive materials near theFabry-Perot optical cavity 12 to limit inductive and capacitiveinterference in the operation of the optical cavity. The Fabry-Perotoptical cavity 12 has a preferred length along the optical Z-axis of 1mm, a width along the optical Y-axis of 1 mm and a thickness along theoptical X-axis of 0.1 mm to 0.2 mm. The optics in the collimating lens160 produces a collimated beam 14 from the optical fiber 118 that isfocused along an optical path 20 substantially parallel to the electrodestructure 10 in the Fabry-Perot optical cavity 12. The modulated opticalsignal generated within the Fabry-Perot optical cavity 12 exits throughthe reflective coating 18 and passes through the collimating lens 160along the optical path 20 which focuses the modulated optical signal onthe optical fiber 120.

Disposed adjacent to the bottom surface of the Fabry-Perot opticalcavity 12 is a probe contact substrate 170 for supporting probingcontacts, such as contact pads and probing tips. The probe contactsubstrate 170 is preferably formed of a non-conductive material, such asalumina, circuit board material or the like. In one embodiment, theprobe contact substrate 170 has apertures 172 formed therein forreceiving electrically conductive probing tips 174. The electricallyconductive probing tips 174 are electrically coupled to the electricallyconductive contacts 30 on the Fabry-Perot optical cavity 12. Theelectrically conductive probing tips 174 may directly contact theelectrically conductive contacts 30 but it is preferable thatelectrically conductive contacts 176 be formed on the upper surface ofthe probe contact substrate 170 that are electrically coupled to theprobing tips 174. The electrically conductive contacts 176 on the probecontact substrate 170 electrically contact the electrically conductivecontacts 30 on the Fabry-Perot optical cavity 12. A conductive adhesive,such as epoxy or the like, is applied to the contacts 30 and 176 forsecuring the probe contact substrate 170 to the Fabry-Perot opticalcavity 12. Alternately, flexible type electrical contacts be disposedbetween the probing pins 174 and the contacts 30. The flexible typecontacts may take the form of electrically conductive elastomers,flexible C-type string contacts, or the like. A mechanical registrationelement would attach the probe contact substrate 170 to the Fabry-Perotoptical cavity 12. In a further embodiment, the apertures 172 andprobing tips 174 may be replaced with protrusions extending from thebottom of the probe contact substrate 170 forming the probing contacts.Electrically conductive material, such as gold plated over a layer ofchromium, is disposed on the bottom surfaces of the protrusions.Electrically conductive vias are formed in the probe contact substrate170 to electrically couple the electrically conductive contacts on theprotrusions to the top surface of the substrate 170.

Optical cavities used as voltage sensing devices, such as theFabry-Perot cavity 12, are high impedance devices. In certain probingapplications it may be preferable to match the impedance at the probe tothe impedance of the device under test 104. As previously described inrelation to FIGS. 3A and 3B, the optional termination resistor 50 may beconnected between the electrically conductive material 28 in theapertures 22 and 24 of the electrode structure 10 or between theelectrically conductive contacts 30 on the exterior surface 32 of theFabry-Perot optical cavity 12. The resistance of the terminationresistor 50 is set to match the impedance of the device under test 104.This allows differential measurements to be made in a defined impedanceenvironment, such as 50 ohms. Terminating the sensing device in theimpedance of the device under test improves signal fidelity by reducingthe reflections that would be caused by impedance mismatches between thedevice under test and the sensing device. The resistance of thetermination resistor 50 may be set to various values to conform tospecific device under test impedance environments. In addition, dampingresistors 179 may be formed on the exterior surface of the Fabry-Perotoptical cavity 12 and coupled in series with each of the electricallyconductive electrodes of the electrode structure 10 or the dampingresistors 179 may be formed on the probe contact substrate 170 in serieswith the electrically conductive contacts 176 be formed on the uppersurface of the probe contact substrate 170.

The collimating lens 160, the Fabry-Perot optical cavity 12 and theprobing contact substrate 170 are disposed within a non-conductivehousing 178, formed from ABS plastic, poly-carbonate, poly-carbonateABS, poly-phenylene sulfide or the like. The housing has a first cavity180 for receiving the collimating lens 160 and a second cavity 182 forreceiving the Fabry-Perot optical cavity 12 and the probe contactsubstrate 170. The housing has an opening 184 extending from the topsurface of the housing to the first cavity 180 to allow the opticalfibers 118 and 120 to be connected to the collimating lens 160. Theinterface between the first and second cavities 180 and 182 defines ashoulder 186. A rib 188 is formed at the bottom of the housing 178 thatprotrudes into the second cavity 182 for supporting the probe contactsubstrate 170, the Fabry-Perot optical cavity 12 and the collimatinglens 160. The first and second cavities 180 and 182 are sized to closelyconform to the lateral dimensions of the collimating lens 160 and theprobe contact substrate 170. Both cavities 180 and 182 are sized toprovide excess vertical clearance for the collimating lens 160 and theprobe contact substrate 170 so as to provide axial movement of theprobing elements within the housing 178. A spring mechanism 190, in theform of elastomeric material, mechanical springs or the like, isprovided in the gaps 192 between the housing 178 and the collimatinglens 160 and the probe contact substrate 170. The housing 178 isdisposed within a probing head shell that provides strain relief for theoptical fibers 118 and 120 in the cable 96 and protection and supportfor the elements within the housing 178.

Acoustic modes are generated in electro-optic material 12 as a result ofpiezoelectric effects of electromagnetic signals on electrodes connectedto the electro-optic material 12. The piezoelectric effect changes thephysical dimensions of the electro-optic material 12 resulting inacoustic distortion that causes optical noise to be imparted in anoptical signal generated by the electro-optic material 12. In an opticalcavity, such as the Fabry-Perot optical cavity 12, the changes in thephysical dimensions of the optical cavity causes variances in theresonance points of the cavity. This results in acoustic distortion thatis imparted as optical noise in the modulated optical return signalgenerated by the Fabry-Perot optical cavity 12. An acoustic dampingmaterial 194 may be applied to the Fabry-Perot optical cavity 12 and/orthe probe contact substrate 170 to minimize the acoustic modes in theFabry-Perot optical cavity 12. The acoustic damping material 194 is madeof an adhesive material, such as epoxy, ultraviolet cured (UV) epoxy,urethane, silicone or the like doped with a ceramic crystallinematerial, such as yttrium-aluminum-garnet or the like. The acousticimpedance of the adhesive material is generally substantially less thanthe acoustic impedance of the electro-optic material 12 in theFabry-Perot optical cavity 12 whereas the acoustic impedance of theceramic crystalline material is substantially higher than theelectro-optic material 12. The blend of the adhesive material and theceramic crystalline material is formulated to match the acousticimpedance of the electro-optic material 12 in the optical cavity. Forthe Fabry-Perot optical cavity 12 having KTP electro-optic material, theacoustic damping material 194 using epoxy as an adhesive has between 25%and 50% by volume of yttrium-aluminum-garnet ceramic crystallinematerial with the preferred volume being 50%. The use of other types ofadhesive material and other types of ceramic crystalline material willalter the volume percentage of the ceramic crystalline material.Further, the use of other types of electro-optic material 12 havingdifferent acoustic impedances requires different percentages or types ofceramic crystalline material. In addition, an optical absorbingmaterial, such as carbon black, may be added to the acoustic dampingmaterial 194 to absorb optical radiation escaping the optical cavity andto prevent extraneous optical radiation from entering the opticalcavity.

The acoustic damping material 194 may be applied to substantially all ofthe surfaces of the Fabry-Perot optical cavity 12 leaving gaps for theoptical signals leaving and entering the collimating lens 160 and forthe electrical connections between the electrically conductive contacts30 on the Fabry-Perot optical cavity 12 and the electrically conductivecontacts 176 on the probe contact substrate 170. In the preferredimplementation, the acoustic damping material 194 is applied tosubstantially all of the outer exposed surfaces of the Fabry-Perotoptical cavity 12 and the probe contact substrate 170. A gap is providedon the top surface of the Fabry-Perot optical cavity for the opticalsignals leaving and entering the collimating lens 160 and the probingcontacts 174 extending from the bottom of the probe contact substrate170 are left exposed.

The probe interconnect housing 98 is plugged into one of theinterconnect receptacles 100 in the measurement instrument 92.Parameters may be set for the variable attenuation signal acquisitionprobing system 90, such as the polarization state of the optical signal,as gain levels of the optical receiver or the like, using controls onthe measurement instrument 92 or via commands sent to the measurementinstrument 92 via an external communications bus. The opticaltransmitter 112 in the signal acquisition probing circuitry 110generates an optical output that is coupled via the optical fiber 118 inthe optical transmission system 96 to the bulk optic collimating lens160. The collimating lens 160 focuses the optical signal on theFabry-Perot optical cavity 12. The user contacts the probing head 94 toa selected test node on the device under test 104 to acquire a signal tobe measured. The measured signal may be a differential signal or asingle signal. The measured signal is coupled through the probingcontacts or pins 174 of the probe contact substrate 170 to the electrodestructure 10 in the Fabry-Perot optical cavity 12. The electrical signalon the electrode structure 10 varies the index of refraction of theelectro-optic material in the Fabry-Perot optical cavity 12 as afunction of the magnitude changes in the electric signal. The changingindex of refraction in the electro-optic material causes correspondingchanges in the reflected optical power from the Fabry-Perot opticalcavity 12. The optically modulated reflected optical power passes out ofthe Fabry-Perot optical cavity 12 into the collimating lens 160 whichfocuses the optically modulated signal onto the end of the optical fiber120. The optical fiber 120 couples the modulated optical signal to theoptical receiver of the signal acquisition probing circuitry 110. Theoptical receiver splits the incoming optical signal and coverts themajority of the optical signal to an electrical signal in a high speedO/E converter 134. The electrical output from the O/E converter 134 isamplified in the amplifier circuitry 136 and coupled to the measurementinstrument 92 via the high speed coaxial interconnect 126. The otherportion of the modulated optical signal is coupled to the slow speed O/Econverter 138. The electrical output from the O/E converter 138represents the average optical power from the Fabry-Perot optical cavity12 and is used for adjusting the output wavelength of the laser 130 tomaintain the optimum reflected optical power from the Fabry-Perotoptical cavity 12.

FIG. 10 illustrates a further embodiment of the variable attenuationsignal acquisition probing system 90 for probing electrical signal on adevice under test 104. The probe interconnect housing 98 is replacedwith an independently powered probe controller 200 and an interconnectadapter 216. The probe controller 200 contains the optical transmitter112 that provides the optical signal to the probe head 94 and theoptical receiver 114 that converts the returning modulated opticalsignal to an electrical signal. The probe controller 200 also includesassociated processing circuitry, such as a micro-controller, memory,ASICs and the like, and a power supply for generating the necessaryvoltages for operating the circuitry within the controller 200. Theprobe controller 200 includes at least a first optical connector 202 forcoupling optical signals to and from the probing head 94. In thepreferred embodiment of the invention, the probe controller 200 includestwo optical connectors 202 with one coupled to an optical transmitter112 in the controller 200 and the other coupled to an optical receiver114. The optical transmission system 204 having one or more opticalfibers, depending on whether the optical signal from the transmitter andthe modulated optical signal from the probe 94 are transmitted throughseparate fibers or through the same fiber, couples the probe controller200 to the probing head 94. The probe controller 200 may include frontpanel controls 206, such as switches 208, knobs 210 and a display 212 toallow for operator inputs to the controller 200. A electrical outputconnector is provided for coupling a wide bandwidth coaxial cable 214having wide bandwidth connectors, such as SMA connectors, from the probecontroller 200 to the measurement instrument 92. The interconnectadapter 216, such as described in U.S. Pat. No. 6,402,549 andincorporated herein in its entirety by reference, includes acorresponding wide bandwidth connector. The interconnect adapter 216 maybe modified to include signal lines for allowing communications betweenthe measurement instrument 92 and the probe controller 200.

FIG. 11 is a more detailed block diagram of the variable attenuationsignal acquisition probing circuitry 110 in the optical circuitrycontroller 200. Like elements from FIG. 7 are labeled the same. Thesignal acquisition probing circuitry 110 is shown in this embodiment hashaving analog bias and TE control circuits 140 and 142 with the feedbackfrom the photodiode 138 bring an analog signal. The optical transmitter112 includes the laser 130 and an optional variable polarizer 220. Theoptical receiver 114 has the same component structure and function aspreviously described. The control circuitry 116 includes the biascontrol and TE control circuitry 140 and 142 as preciously described.The data and control bus 146 couples the micro-controller 144 to thevariable polarizer, the front panel controls 206, the display device 212and the D/A converter 150. A power supply 224 provides voltage power tothe circuits within the optical circuitry controller 200.

A user sets the attenuation factor for the variable attenuation signalacquisition probing system 90 using the front panel controls 206. Thechanges in the front panel controls 206 are interpreted by themicro-controller 144 which sets the polarization state for the variablepolarizer 220. The optical output from the laser 130 is coupled throughthe variable polarizer 220 to the Fabry-Perot cavity 12 in the probinghead 94. The Fabry-Perot cavity 12 senses the electrical signal from thedevice under test 104 and couples a modulated return optical signalrepresentative of the sensed electrical signal. The modulated returnoptical signal is converted to and amplified by the optical receiver 114and coupled to the measurement instrument 92. Changing the polarizationstate of the optical signal from the laser 130 results in a change inthe attenuation factor for the variable attenuation probing system 90.

A variable attenuation signal acquisition probing system has beendescribed where an optical cavity is used to acquire an electricalsignal from a device under test. The optical cavity receives an opticalsignal from an optical transmitter via an optical transmission systemand generates a modulated optical signal derived from the device undertest electrical signal creating an electromagnetic field distribution inelectro-optic material in the optical cavity that overlaps the opticalpath of the optical signal propagating in the electro-optic material andvaries the index of refraction of the electro-optic material in theoptical path. The modulated optical signal is coupled to an opticalreceiver via the optical transmission system which converts themodulated optical signal to an electrical signal. The electrical signalis coupled to measurement test instrument for processing and displayingof the electrical signal. The variable attenuation signal acquisitionprobing system included control circuitry for controlling the opticalpower level and wavelength of the optical signal from the opticaltransmitter and the gain of the output electrical signal from theoptical receiver.

The optical cavity is preferably a Fabry-Perot optical cavity havingelectrically conductive electrodes disposed in the optical cavityparallel to one of the optical axes of the cavity and generally parallelto the received optical signal propagating within the optical cavity.The electrically conductive electrodes are made in the optical cavity byforming parallel apertures in the optical cavity having electricallyconductive material disposed therein.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

1. A variable attenuation signal acquisition probing system usingelectro-optic detection for sensing an electrical signal from a deviceunder test comprising: an optical transmitter generating a variablepolarized, tunable, coherent optical signal; an optical receivergenerating an output electrical signal; circuitry for controlling theoptical power level and wavelength of the tunable, coherent opticalsignal from the optical transmitter and the gain of the outputelectrical signal from the optical receiver; an optical cavity havingoptically reflective material disposed on opposing surfaces of anelectro-optic material with the tunable, coherent optical signalpropagating through at least one of the optically reflective materialsand within the electro-optic material; first and second electricallyconductive electrodes receiving the electrical signal from the deviceunder test with each of the first and second electrically conductiveelectrodes having an apertures formed in at least a portion of theelectro-optic material generally parallel to the received optical signalpropagating within the electro-optic material and electricallyconductive material disposed within each of the first and secondapertures; an optical transmission system optically coupled to theoptical transmitter, optical receiver and one of the opticallyreflective materials of the optical cavity providing the optical signalfrom the optical transmitter to the optical cavity and providing amodulated optical signal to the optical receiver representing theelectrical signal from the device under test derived from the deviceunder test electrical signal creating an electro-magnetic fielddistribution in the electro-optic material that overlaps the opticalpath of the optical signal propagating in the electro-optic material andvaries the index of refraction of the electro-optic material in theoptical path; and a variable polarizer receiving the tunable, coherentoptical signal and varying the polarization state of tunable, coherentoptical signal to vary the magnitude of the output electrical signal ofthe optical receiver.
 2. The variable attenuation signal acquisitionprobing system as recited in claim 1 further comprising a resistorcoupled between the first and second electrically conductive electrodes.3. The variable attenuation signal acquisition probing system as recitedin claim 1 further comprising a resistor to each of the first and secondelectrically conductive electrodes.
 4. The variable attenuation signalacquisition probing system as recited in claim 1 wherein the opticalcavity further comprises electrically conductive contacts formed on anat least one exterior surface of the optical cavity with the one of theelectrically conductive contacts electrically coupled to the firstelectrically conductive electrode and the other electrically conductivecontact electrically coupled to the second electrically conductiveelectrode.
 5. The variable attenuation signal acquisition probing systemas recited in claim 4 further comprising a resistor coupled between theelectrically conductive contacts.
 6. The variable attenuation signalacquisition probing system as recited in claim 4 further comprising aresistor coupled to each of the electrically conductive contacts.
 7. Thevariable attenuation signal acquisition probing system as recited inclaim 1 wherein the received optical signal propagates generallyparallel to at least a first optical axis in the electro-optic materialwith the first and second electrically conductive electrodes generallyparallel to same optical axis.
 8. The variable attenuation signalacquisition probing system as recited in claim 1 wherein theelectro-optic material has X, Y, and Z optical axes and correspondingcrystal faces orthogonal to the respective X, Y, and Z optical axes withthe optical cavity further comprising the opposing optically reflectivematerials being disposed on the Y-crystal face and the first and secondelectrically conductive electrodes being orthogonal to the Y-crystalface of the electro-optic material.
 9. The variable attenuation signalacquisition probing system as recited in claim 1 wherein theelectro-optic material has X, Y, and Z optical axes and correspondingcrystal faces orthogonal to the respective X, Y, and Z optical axes withthe optical cavity further comprising the opposing optically reflectivematerials being disposed on the X-crystal face and the first and secondelectrically conductive electrodes being orthogonal to the X-crystalface of the electro-optic material.
 10. The variable attenuation signalacquisition probing system as recited in claim 1 wherein theelectro-optic material has X, Y, and Z optical axes and correspondingcrystal faces orthogonal to the respective X, Y, and Z optical axes withthe optical cavity further comprising the opposing optically reflectivematerials being disposed on the Z-crystal face and the first and secondelectrically conductive electrodes being orthogonal to the Z-crystalface of the electro-optic material.
 11. The variable attenuation signalacquisition probing system as recited in claim 1 wherein the opticalcavity comprises a Fabry-Perot optical cavity.
 12. The variableattenuation signal acquisition probing system as recited in claim 1wherein the optical transmission system further comprises an opticaldirectional coupler having a first port optically coupled to the opticaltransmitter, a second port optically coupled to the optical receiver anda third port optically coupled to one end of an optical fiber with theother end of the optical fiber optically coupled to one of the opposingoptically reflective materials of the optical cavity.
 13. The variableattenuation signal acquisition probing system as recited in claim 12further comprising a collimating lens optically coupled to the opticalfiber with the collimating lens disposed adjacent to one of the opposingoptically reflective materials of the optical cavity.
 14. The variableattenuation signal acquisition probing system as recited in claim 13wherein the optical directional coupler is a polarization maintainingoptical directional coupler having the first port optically coupled tothe optical transmitter via a polarization maintaining optical fiber andthe third port of the polarization maintaining optical directionalcoupler coupled to the collimating lens via a polarizing maintainingoptical fiber.
 15. The variable attenuation signal acquisition probingsystem as recited in claim 1 wherein the optical transmission systemfurther comprises: a polarizing maintaining optical fiber opticallycoupled to the optical transmitter; a collimating lens optically coupledto the polarizing maintaining optical fiber with the collimating lensdisposed adjacent to one of the opposing optically reflective materialsof the optical cavity; and an optical fiber optically coupled to thecollimating lens and the optical receiver.
 16. The variable attenuationsignal acquisition probing system as recited in claim 1 wherein theoptical receiver further comprises: at least a firstoptical-to-electrical converter coupled to receive the modulated opticalsignal from the optical cavity via the optical transmission system andgenerating an electrical signal; and an amplifier coupled to receive theelectrical signal from the optical-to-electrical converter andgenerating an electrical signal representative of the electrical signalfrom the device under test.
 17. The variable attenuation signalacquisition probing system as recited in claim 16 wherein the opticalreceiver further comprises an optical beam splitter optically coupled toreceive the modulated optical signal from the optical cavity andoptically coupling a first portion of the modulated optical signal tothe first optical-to-electrical converter and a second portion to asecond optical-to-electrical converter.
 18. The variable attenuationsignal acquisition probing system as recited in claim 17 wherein thesecond optical-to-electrical converter generates an electrical signalthat is coupled to the control circuitry for varying the wavelength ofthe coherent optical signal of the optical transmitter to maintain anoptimum modulated reflected power from the optical cavity.
 19. Thevariable attenuation signal acquisition probing system as recited inclaim 17 wherein the optical bean splitter optically couples greaterthan ninety percent of the modulated optical signal to the firstoptical-to-electrical converter and less than ten percent to the secondoptical-to-electrical converter.
 20. The variable attenuation signalacquisition probing system as recited in claim 1 further comprising anacoustic damping material substantially covering the optical cavity tominimize acoustic modes in the optical cavity.